This invention relates to semiconductor devices and circuits and methods for driving LEDs in lighting and display applications.
LEDs are increasingly being used to replace lamps and bulbs in lighting applications including providing white light as a backlight in color liquid crystal displays (LCD) and high definition televisions (HDTV). To backlight a color LCD panel, the LED strings may comprise white LEDs or combinations of red, green, and blue LEDs adjusted to produce white light with controllable color temperature. While these LEDs may be used to uniformly light the entire display, the performance, contrast, reliability, and power efficiency of the display are improved by employing multiple strings of LEDs, each driven to different current and brightness levels corresponding to that portion of the display the particular LED string illuminates. The term “local dimming” refers to backlighting systems capable of such non-uniform backlight brightness. Power saving can be as high as 50% over that of LCDs employing uniform backlighting. Using local dimming, LCD contrast ratios can approach that of plasma TVs.
To control the brightness and uniformity of the light emitted from each string of LEDs, special electronic driver circuitry must be employed to precisely control the LED current and voltage. For example, a string of “m” white LEDs connected in series requires a voltage equal to approximately 3.1 to 3.5 (typically 3.3) times “m” to operate consistently. Supplying this requisite voltage to a LED string generally requires a step-up or step-down voltage converter and regulator called a DC-to-DC converter or switch-mode power supply (SMPS). When a number of LED strings are powered from a single SMPS, the output voltage of the power supply must exceed the highest voltage required by any of the strings of LEDs. Since the highest forward voltage required cannot be known a priori, the LED driver IC must be intelligent enough to dynamically adjust the power supply voltage using feedback. If two or more power supply voltages are required, more than one feedback signal is required.
In the case of RGB backlighting, the voltage feedback requirement is even more complex because red, green and blue LEDs have significantly different forward voltages and cannot share a common power supply rail. Instead, RGB LED strings require three different supply voltages, +VRLED, +VGLED, and +VBLED, respectively, each with separate feedback signals to dynamically adjust their respective power supply voltages to the proper level. For example, a string of 30 red LEDs in series requires a supply over 66V to operate properly, while 30 blue LEDs may require a supply over 96V, and 30 green LEDs requires a supply of more than 108V.
In addition to providing the proper voltage to the LED strings, the backlight driver must precisely control the current ILED conducted in each string to a tolerance of ±2%. Accurate current control is necessary because the brightness of an LED is proportional to the current flowing through it, and any substantial string-to-string current mismatch will be evident as a variation in the brightness of the LCDs. Aside from controlling the current, local dimming requires precise pulse control of LED illumination, both in timing and duration, in order to synchronize the brightness of each backlight region, zone, or tile to the corresponding image in the LCD screen.
Another complication is that the color temperature of white LEDs varies with current. As an example, a string of white LEDs conducting 30 mA for 100% of the time is ideally equivalent in brightness to the same LED string carrying 60 mA pulsed on and off at a 50% duty factor. Even at the same brightness, however, the color temperature will not be the same. So accurately setting and maintaining current in each string is critical to achieving a uniform white backlight for a color LCD panel.
In the case of RGB backlighting, balancing current is even more complex, since the luminosity, i.e. the light output or brightness, of red, blue and green LEDs differs substantially. Red LEDs, for example, produce less light for the same LED current than blue LEDs. The difference is understandable since the semiconductor materials and the manufacturing process used to make LEDs of different colors differ significantly.
As will be shown in this background section, the known solutions to local dimming limit display brightness and suffer from high solution costs. For example, early attempts to integrate LED driver control circuitry with multiple channels of high-voltage current sink transistors were problematic because a mismatch in the forward-voltage of the LED strings resulted in excessive power dissipation and overheating. Attempts to minimize power dissipation by lowering LED currents and limiting the number of LEDs in a string (for better channel-to-channel voltage matching) proved uneconomical, requiring more LEDs and a greater number of channels of LED drive. As a result, the fully integrated approach to LED backlight drive has been limited to small display panels or very expensive “high-end” HDTVs.
Subsequent attempts to reduce overall display backlight costs using multichip approaches have sacrificed necessary features, functionality, and even safety.
For example, the multichip solution to driving LEDs shown in FIG. 1 comprises an interface IC 6 driving multiple discrete current sink DMOSFETs 4 and high-voltage protection devices 3. The backlight system comprises sixteen LED strings 2A-2Q (referred to collectively as LED strings 2) with each of LED strings 2A-2Q containing “m” series-connected LEDs, ranging in length from 2 to sixty LEDs. (Note that the letter “O” has been omitted in the series 2A-2Q to avoid confusion with the number zero.) Each LED string has its current controlled by one of discrete current sink DMOSFETs 4A-4Q, respectively. Interface IC 6 sets the current in each LED string in response to instructions from a backlight microcontroller (μC) 7 communicated through a high speed, expensive, SPI bus interface 11. Microcontroller μC 7 receives video and image information from a scalar IC 8 in order to determine the proper lighting levels needed for each LED string.
As shown, every LED string 2A-2Q is powered by a common LED power supply rail 12, generated by a switch-mode power supply (SMPS) 9, having a voltage +VLED generated in response to a current-sense feedback (CSFB) signal 10 through feedback from interface IC 6. Supply voltages vary with the number of LEDs “m” connected in series and may range from 35 volts for ten LEDs up to 150 volts for strings of 40 LEDs. SMPS 9 may be powered from the AC mains or alternatively from another input such as a +24V input.
SMPS 9 typically comprises a flyback converter operating in hard switching or in quasi-resonant mode. Forward converters and Cuk converters, while applicable, are generally too expensive and unnecessarily complex to serve the cost-sensitive display and TV markets. In the event that SMPS 9 is powered from a +24V input, its operation depends on the number of LEDs connected in series. If the forward voltage of the LED string is less than 24V, e.g. a series connection of less than 7 LEDs, SMPS 9 can be realized using a Buck-type switching regulator. Conversely, if the forward voltage of the LED string is greater than 24V, e.g. a series connection of more than 8 LEDs, then SMPS 9 can be realized using a boost-type switching regulator.
Regardless of its input voltage, the proper generation of the CSFB signal 10 is critical to achieving reliable operation for a display's LED backlight. If the feedback signal is incorrect, the LED supply voltage +VLED may be too high or too low. If the LED supply voltage is too high, excess power dissipation will occur in the current sink DMOSFETs 4A-4Q. If the LED supply voltage is too low, the LED strings requiring the highest current will not illuminate at the prescribed level, if at all.
To implement the CSFB function, accurate monitoring of a LED string's forward-voltage requires electrical access to the drain of the current sink DMOSFETs 4A-4Q, which for multichip implementations can be particularly problematic, resulting in additional package pins and added component cost.
Current sink DMOSFETs 4A-4Q are realized using discrete DMOSFETs to avoid overheating. Additional discrete MOSFETs 3A-3Q, typically high voltage discrete MOSFETs, are optionally employed to clamp the maximum voltage present across the current sink DMOSFET 4, especially for operation at higher voltages, e.g. over 100V.
Each of components 3A-3Q is a discrete device in a separate package, requiring its only pick-place operation to position and mount it on its printed circuit board. A current sink DMOSFET, a clamping MOSFET (if any), and its associated LED string are commonly referred to as a “channel.”
Each set of discrete MOSFETs 3A-3Q and DMOSFETs 4A-4Q, along with its corresponding string of white LEDs, is repeated “n” times for an n-channel LED driver system. For example, in addition to SMPS module 9, a 16-channel backlight system requires 34 components, namely a microcontroller, a high-pin-count LED interface IC, and 32 discrete MOSFETs, to facilitate local dimming in response to video information generated from scalar IC 8. The solution is complex and expensive.
In some cases, it is desirable to split LED power into more than one power supply, e.g. to reduce the power dissipation in any one supply and its components, but prior art LED interface ICs cannot support multiple independent feedback signals. In the case of RGB backlit displays, the solution is even more complex and expensive. Since existing and prior art LED drivers and controllers include only a single CSFB signal per integrated circuit, independently regulating three different power supplies requires three separate LED interface ICs along with three separate power supplies, making today's RGB backlighting solutions prohibitively expensive.
In either case, the assembly of a large number of discrete components, i.e. a high build of materials (BOM) count, results in expensive PCB assembly, further exacerbated by the high package cost of high pin count package 6. The need for such a large number of pins is illustrated in FIG. 2A, illustrating greater circuit detail for an individual channel of an LED drive system. As shown, each channel includes a string of “m” series connected LEDs 21, a cascode-clamp MOSFET 22 with an integral high-voltage diode 23, a current sink MOSFET 24, and a current-sensing I-Precise gate driver circuit 25.
The active current sink MOSFET 24 is a discrete power MOSFET, preferably a vertical DMOSFET, having a gate, source and drain connection. I-Precise gate driver circuit 25 senses the current in current sink MOSFET 24 and provides it with the requisite gate drive voltage to conduct a precise amount of current. In normal operation, current sink MOSFET 24 operates in its saturated mode of operation, controlling a constant level of current independent of its drain-to-source voltage. As a result of the simultaneous presence of drain voltage and current, power is dissipated in MOSFET 24.
Continuous measurement of the drain voltage of current sink MOSFET 24 is required for two purposes—to detect the occurrence of shorted LEDs in an LED fault circuit 27 and to facilitate feedback to the system's SMPS through CSFB circuit 26. The signal generated by CSFB circuit 26 is critical to dynamically adjust +VLED to the proper voltage, high enough to guarantee every LED string is illuminated but low enough to avoid excess voltage impressed upon the current sink DMOSFET 24 resulting in unwanted power dissipation. With only one CSFB signal, it is not possible to power the LEDs from more than one power supply, i.e. to split the power requirements in two to reduce the size, cost and heating in the SMPS.
Current sink MOSFET 24 requires three connections to the control IC, specifically the source for current measurement, the gate for biasing the device, and the drain for fault and feedback sensing. These three connections per channel are depicted crossing the discrete-to-IC interface 28. Even in FIG. 2B where a cascode clamp MOSFET 22 is eliminated and current sink MOSFET 24 must sustain high voltages, illustrated by HV integral diode 23, each channel still requires three pins per channel crossing interface 28. This three-pin per channel requirement explains the need for high-pin count interface IC 6 shown in FIG. 1. For a sixteen-channel driver, the need for three pins per channel uses 48 pins for the outputs. Including the SPI bus interface, analog functions, power supplies and more, a costly 64 or 72-pin package is necessary. Worse yet, many TV printed circuit board assembly houses are incapable of soldering packages with a pin pitch any smaller than 0.8 or 1.27 mm. A 72-pin package with a 0.8 mm pin pitch requires a 14×14 mm plastic body to accommodate the peripheral linear edge needed to fit all the pins.
One significant issue with the multichip structure shown in FIG. 1 is that temperature sensing circuitry in interface IC 6 can only detect the temperature of the interface IC itself, where no significant power dissipation is occurring. Unfortunately, the significant heat is being generated in discrete current sink DMOSFETs 4, where no temperature sensing is possible. Without temperature sensing, any one of the current sink MOSFETs 4A-4Q could overheat without the system being able to detect or remedy the condition.
In summary, today's implementations for LED backlighting of LCD panels with local dimming capability suffer from numerous fundamental limitations in cost, performance, features, and safety.
Highly integrated LED driver solutions require expensive large area dice packaged in expensive high pin count packages, and concentrate heat into a single package, limiting the driver to lower currents, due to power dissipation resulting from the linear operation of the current sink MOSFETs, and lower voltages, due to power dissipation resulting from LED forward-voltage mismatch, exacerbated for greater numbers of series connected LEDs.
Multi-chip solutions combining an LED controller with discrete power MOSFETs require BOM counts and even higher-pin-count packaging. Having nearly triple the pin count of fully integrated LED drivers, a sixteen channel solution can require 33 to 49 components and a 72 pin package as large as 14 mm×14 mm. Moreover, discrete MOSFETs offer no thermal sensing or protection against overheating. With only one feedback signal, these LED drivers cannot power two or more LED power supplies without including additional interface ICs, adding cost and complexity.
Similarly, expanding the use of these existing LED driver and interface ICs to RGB backlighting requires even a higher BOM count, including three large high-pin count packages and all the associated discrete MOSFETs.
What is needed for a cost effective and reliable backlight system for TV's with local dimming is a new semiconductor chip set that eliminates discrete MOSFETs, provides a low overall package cost, minimizes the concentration of heat within any component, facilitates over-temperature detection and thermal protection, protects low voltage components from high voltages and against shorted LEDs, flexibly scales to accommodate different size displays, and maintains precise control of LED current and brightness.
Ideally, a flexible solution would be scalable to accommodate a varying number of channels, feedback signals, power supplies, and display panels of different sizes without requiring custom integrated circuits.